The present invention relates to a process for the preparation of polybenzoxazole or polybenzothiazole filaments and fibers.
Fibers prepared from polybenzoxazole (PBO) and polybenzothiazole (PBT) (hereinafter referred to as PBZ or polybenzazole polymers) may be prepared by first extruding a solution of polybenzazole polymer in a mineral acid (a polymer "dope") through a die or spinneret to prepare a dope filament. The dope filament is then drawn across an air gap, washed in a bath comprising water or a mixture of water and a mineral acid, and then dried. If multiple filaments are extruded simultaneously, they may then be combined into a multifilament fiber before, during, or after the washing step.
Dried polybenzazole fibers prepared by such a process are known to have a tensile modulus in the range of from about 20 to about 25 Msi. For certain applications however, a higher tensile modulus is desirable. Accordingly, processes are known for preparing fibers with a higher tensile modulus, by washing and drying a spun fiber, and then heat-treating the fiber at a temperature of at least about 350.degree. C. Such processes are described, for example, in U.S. Pat. No. 5,288,445. However, such processes typically require the fiber to have been dried to a residual moisture content of less than about 2 percent and a residual solvent content of less than about 2 percent prior to heat-treatment, in order to obtain a fiber with good physical properties. Further, such heat-treating processes add to the time and expense necessary to prepare a suitable fiber. In addition, such heat-treatment processes typically result in a measurable decrease in the tensile strength of the fiber. While such fibers may be useful for certain applications, further improvement in the physical properties of the fiber is desirable.